In the past, in optical communications, a wavelength division multiplexing (WDM) that causes a wavelength multiplexing light obtained by multiplexing a plurality of signal lights of different wavelengths to pass through a transmission line such as an optical fiber and thereby transmits large volumes of information at one time has been known. In WDM, in general, by increasing the number of wavelengths or decreasing channel spacing, the capacity of information that is transmitted at one time is increased. As a transmission method for increasing the transmission capacity of information more efficiently, polarization multiplexing is known. Polarization multiplexing transmits two independent pieces of data information at one time by using a polarization multiplexing signal light obtained by combining two signal lights whose polarization planes are orthogonal to each other, and coherent reception is performed at a receiving end and polarization separation and waveform correction are performed by a digital signal processor (DSP).
In general, an optical transmission apparatus that adopts WDM and polarization multiplexing is provided with an optical amplifier that amplifies a wavelength multiplexing light to be transmitted. When a wavelength multiplexing light obtained as a result of polarization multiplexing signal lights having been multiplexed is input to the optical amplifier, the wavelength multiplexing light is amplified by using the optical amplifier, and the amplified wavelength multiplexing light is output to a transmission line such as an optical fiber.
Here, at the time of initial introduction of the optical transmission apparatus, sometimes only part of all the wavelengths belonging to the amplification band of the optical amplifier is used as a wavelength for amplifying the polarization multiplexing signal light in the wavelength multiplexing light and the other wavelengths are not used. In this case, the optical power per channel of the optical amplifier may be increased, and, due to the influence of the gain of the unused wavelength band, a phenomenon in which the gain wavelength characteristics of the optical amplifier are distorted may occur. An increase in the optical power per channel of the optical amplifier causes signal degradation by a deviation from the value of optimum optical power, and the phenomenon in which the gain wavelength characteristics of the optical amplifier are distorted is called a spectral hole burning phenomenon. The occurrence of the spectral hole burning phenomenon becomes a factor in deteriorating the transmission quality of a signal light that is output from the optical amplifier.
Thus, a technique of inputting a non-signal light to the unused wavelength band of the amplification band of the optical amplifier is being studied. In this technique, by multiplexing a non-signal light of a wavelength of the wavelengths belonging to the amplification band of the optical amplifier, the wavelength being different from the wavelength of a polarization multiplexing signal light, and the polarization multiplexing signal light, a wavelength multiplexing light is generated, and the wavelength multiplexing light thus generated is input to the optical amplifier. As the non-signal light, a continuous wave (CW) light having a single polarization plane is used. As described above, by inputting the non-signal light to the unused wavelength band of the amplification band of the optical amplifier, it is possible to suppress an increase in the optical power per channel of the optical amplifier and avoid the occurrence of the spectral hole burning phenomenon.
Japanese National Publication of International Patent Application No. 2008-510388 and Japanese National Publication of International Patent Application No. 2009-530943 are examples of related art.